1. Field of the Invention
This invention relates generally to multiple drive marine vessels and, more particularly, to control apparatus for adjusting and equalizing the angular trim position of plural outboard motors or other stern drives on boats.
2. Description of the Prior Art
Many techniques are known for individually tilting or trimming plural-outboard motors or other stern drives on boats to a desired angular trim position. The trimming of drives of boats is normally effected by an operator controlled, three position switch, i.e., up, down and off, which individually varies the trim or angular positioning of a drive during boat operation. Separate switches are usually provided for each drive. Each switch can also raise and lower the drives for clearance purposes. A dash mounted trim indicator may be read by the operator for determining the trim of a drive and, on a multiple drive boat, the switches may be read and compared for facilitating the equalization of the trim angle of the drives.
Trimming is carried out within a trim range defined by full down drive position and some higher trim limit position. Changes in the angle of propeller thrust can cause the boat bow to raise or lower and thus enables an operator to select a proper and more efficient angle for different loads or different water conditions or for operating the boat on plane. In addition, tilting or raising the drive or drives up past the trim range for clearance purposes enables safer running in shallow water and also facilitates launching or trailering. Note U.S. Pat. Nos. 3,434,449 and 3,641,965 for descriptions of various approaches to effecting trim.
In dual drive boats, two independent propulsion systems are utilized along with two independent trimming devices. While drive tilt systems generally move both drives simultaneously, the criticality of trim angles in high performance drives has generally required independent control of trim angles. After each occurrence of trim adjustment, the trim angle of each of the two drives will not necessarily be identical. A secondary trim adjustment of one of the drives must usually be made by the operator who is required to simultaneously observe and compare two dash mounted trim indicators in order to properly synchronize the trim of the two drives.
An example of the above described drive control system is disclosed in U.S. Pat. No. 4,310,320 to Pitchford. In Pitchford, a tie bar mechanically connects the housings of two stern drives and allows the drives to assume different trim positions within a trim limit range. Since damage can be incurred if the trim cylinders are operated independently to move one drive up or down for a substantial distance while the other drive remains stationary, outside the narrow trim range the control automatically moves the drives simultaneously. Within the trim range, the boat operator must still independently control and adjust the trim angle by monitoring trim indicators associated with each drive. Thus, while Pitchford provides conjoint movement for some tilt angles, the problem of trim adjustment for optimal performance while running has not been resolved and still requires operator intervention.